Sandwich panels (e.g., a core structure sandwiched between two layers of material, or skins) are often used in the construction of aircraft, because they have high strength to weight ratios. Depending on the specific location and application of a sandwich panel in an aircraft, one or more round inserts may be required to be inserted within or through a sandwich panel in order to affix one or more other structures or fasteners to the panel. Round inserts may be used to transfer localized loads (e.g., via a pin, bolt, screw, joint, or other structure) to the sandwich panel, such as to fasten the sandwich panel to another structure, join multiple sandwich panels to each other, and/or attach one or more external objects to the sandwich panel. For example, round inserts may provide a channel for wire bundles to be passed from one side of the sandwich panel to the other, or round inserts may be configured to receive a pin or bolt or other fastener in order to secure another panel or object to the sandwich panel, via the round insert. In the aerospace industry, such sandwich panels and round inserts may be used to assemble the interior main structure and/or secondary structures of the aircraft, and/or may be used to form floor boards, wall panels, galleys, stow bins, overhead compartments, lavatories, and/or other structures within the aircraft. Such sandwich panels and round inserts are also used in other industries.
FIGS. 1 and 2 illustrate a conventional round insert 10 installed in a sandwich panel 11, shown schematically in cross-section. Sandwich panel 11 may include a core 13 sandwiched between a first skin 15 and a second skin 17. First skin 15 and second skin 17 may be rigid or semi-rigid skins, and are typically relatively thin compared to core 13, which is typically formed of a lightweight material. Conventional round insert 10 may be inserted into a circular bore 19 formed in sandwich panel 11, which may be a blind bore 21 (FIG. 1) or a through-bore 23 (FIG. 2). Blind bore 21 may extend through one of the skins (e.g., first skin 15) and into the core 13, towards the other skin (e.g., second skin 17), whereas through-bore 23 may extend entirely through first skin 15, second skin 17, and core 13. As shown in FIGS. 1 and 2, a flange portion 27 of conventional round insert 10 may be substantially flush with one of more of first skin 15 and second skin 17, or, as shown in FIG. 3, flange portion 27 of conventional insert 10 may lay on top of (e.g., on an outer surface of) first skin 15 or second skin 17.
In conventional techniques, an adhesive material, such as a potting compound or epoxy, is injected through potting holes, or vents, in conventional round insert 10 to fill a gap or space 29 between conventional insert 10 and core 13 of sandwich panel 11. The adhesive material, once fully cured, serves to secure the insert in place within bore 19 of sandwich panel 11, substantially preventing relative movement of conventional round insert 10 with respect to sandwich panel 11 and retaining conventional insert 10 within circular bore 19 (e.g., resisting pull-out, rotation, and lateral movement of conventional round insert 10) once the adhesive compound dries, solidifies, and/or cures. However, the use of such adhesive compounds is labor-intensive and may take a significant period of time to cure, such as 2-4 hours or more. During this time, the conventional round insert is not stable within the sandwich panel, and loads cannot be applied to conventional round insert 10, which limits efficiency of the manufacturing process. Furthermore, often, conventional round inserts 10 must be held in place during the cure time (e.g., with the application of masking tape to hold conventional round insert 10 in place), and even with such mitigating techniques, the insert may shift during curing.
In some methods, if excess adhesive material has squeezed out around conventional round insert 10, it must be removed from conventional round insert 10, so that secondary objects (e.g., brackets) may be substantially flush with the insert and sandwich panel. Conventional round inserts 10 and methods of using and/or installing the same also may suffer from “high inserts” or “low inserts,” where the conventional round insert is ultimately positioned too far out of the circular bore (“high”) or too far into the circular bore (“low”), respectively (e.g., the outer surface of conventional round insert 10 may be too “high,” sticking out of the circular bore, or too “low,” too far recessed within the circular bore, in either case thereby creating an un-smooth interface with the sandwich panel skin). Such imperfect positioning may be the result of shifting during cure time and/or poor initial placement, and may prevent the proper positioning of the bracket or other secondary device secured via conventional round insert 10. Expansion forces from the adhesive material expanding during curing may also contribute to defects, such as shifting of conventional round insert 10, and may cause visible “mark-off” on the opposite side of the sandwich panel, often an unacceptable result in the finished product.
The use of adhesive material to bond a conventional round insert 10 within the circular sandwich panel bore may disadvantageously prevent automation of the manufacturing technique. Furthermore, installation of an incorrect type of conventional round insert 10 may be difficult and time-consuming to correct, especially in techniques where the conventional round inserts are secured with adhesive material. Many different types of conventional round inserts may be installed within a single sandwich panel, and of the different types of conventional round inserts, they may be interchangeable within the same size circular bores formed in the sandwich panel, thereby making the installation of an incorrect conventional round insert fairly commonplace. Adhesive materials used in conventional techniques also may fail, even after curing, which may result in pull-out of conventional round insert 10 from the circular bore or spinning of the insert within the circular sandwich panel bore when torque forces breakage of the bond between the conventional round insert and the adhesive compound. Such issues with conventional round inserts 10 and methods of installing the same within a sandwich panel are on-going and problematic in a variety of industries.